This invention relates to a composition and method for use in inhibiting acrylonitrile polymerization, particularly in quench columns of acrylonitrile producing systems.
Acrylonitrile is commercially produced in systems with the Sohio process. In a commercial acrylonitrile system utilizing this process, the reactor feeds are propylene, ammonia and compressed air. The propylene and ammonia are vaporized, then combined with the air and fed to a fluidized bed catalytic reactor. Precise ratios of the three feeds are maintained for optimum yield. The catalyst in the reactor vessel is a powder, which is maintained in a turbulent fluid state through the velocity of the air feed. The three mix together in the reactor and react on the surface of the fluidized catalyst. A set of complex exothermic reactions takes place, thereby forming the following products: acrylonitrile, hydrogen cyanide, carbon dioxide, carbon monoxide, acetonitrile, acrolein, acrylic acid, water, other higher nitriles, aldehydes, ketones, acetic acid and a number of miscellaneous unknown organic compounds. Conversion of the three feeds is less than 100%, thus unreacted propylene, ammonia, oxygen and nitrogen are contained in the reactor effluent gas. A portion of the heat of the exothermic reaction is removed by sets of steam coils which generate and superheat waste steam at approximately 600 psig. Reactor effluent gas passes through cyclones, which remove catalyst fines from the gas. The gas is then further cooled in a reactor effluent cooler, which is comprised of a shell and tube exchanger using boiler feedwater as the cooling source.
As the gas leaves the reactor effluent cooler, it then enters a quench column. The quench column cools the reactor effluent by contacting it with a recirculating water stream. Most of the water vapor and small amounts of organic vapors in the reactor effluent are condensed in the quench column. The quench column bottoms are cooled and circulated back to the quench column. The quench column can contain internal trays or packing to provide intimate contact of upflowing gas with downflowing water. Sulfuric acid is injected into the recirculating quench water to neutralize unreacted ammonia in the reactor effluent. The excess quench water is roughly equal to the amount of water produced by the reactor and is fed to the wastewater column where acrylonitrile and hydrogen cyanide are recovered. Wastewater column bottoms are cooled and neutralized, mixed with other plant waste streams, clarified and injected into the wastewater injection well.
The quench column effluent gas is then directed to an absorber where chilled water is used to absorb acrylonitrile, hydrogen cyanide and other organics from the gas. Absorber bottoms are fed to a recovery column where a crude acrylonitrile product is taken overhead. The crude acrylonitrile product is then purified using a series of distillation columns, referred to as the purification section. The first column (heads column) removes hydrogen cyanide, while the second column (drying column) removes water. The last column (product column) takes a pure acrylonitrile monomer product from a side-draw near the top of the column. Heavy ends are rejected from the product column bottoms.
The acrylonitrile can polymerize in the quench column. More specifically, as the reactor effluent gas is passed through the quench column, a portion of the acrylonitrile contained in the gas polymerizes and is absorbed into the recirculating quench water. The amount of acrylonitrile that polymerizes in the quench column represents an undesirable net product loss for the acrylonitrile plant. For example, in an uninhibited quench column, between about 2% to 5% of the total acrylonitrile produced by the reactor is lost due to polymerization in the quench column.
Known polymerization inhibitors for acrylates include phenothiazine, hydroquinone, the methyl ether of hydroquinone, benzoquinone, and methylene blue. Of primary interest is Japanese Patent No. 47-18820 which discloses the use of dialkylhydroxylamine of generic structure ##STR3## (with R and R' as its alkyl radicals), singly or together with other sundry polymerization inhibitors, to inhibit polymerization of unsaturated compounds of generic structure ##STR4## (where R stands for hydrogen or methyl radical and R' and R" for alkyl radicals). Also, May in U.S. Pat. No. 3,408,422 discloses a process for stabilizing ethylenically unsaturated polyesters and a composition stabilized against premature gelatin comprising (1) a hydroxy-containing ethylenically unsaturated polyester of a glycidyl polyether of a polyhydric phenol and an ethylenically unsaturated monocarboxylic acid, and (2) a hydroxylamine compound.
Phenylenediamines alone or with oxygen are known in the art as polymerization inhibitors in acrylate systems. Otsuki et al. in U.S. Pat. No. 3,674,651 discloses a process for inhibiting the polymerization of acrylic acid using a combination of diphenylamine or its derivatives and an oxygen-containing gas, or mixtures of diphenylamine or its derivatives with benzoquinone and/or hydroquinone mono-methyl-ether and an oxygen-containing gas. Wilder, in U.S. Pat. No. 4,016,198, discloses a method of inhibiting polymerization of unsaturated carboxylic acid esters and improved unsaturated carboxylic acid ester compositions comprising incorporating into the ester composition a combination of polyalkyleneamines and certain N-aryl-o or p-phenylenediamines. Also, Mullins in U.S. Pat. No. 4,017,544 discloses the use of a class of N-(nitroalkyl)-N'-phenyl-p-phenylenediamines to inhibit the polymerization of unsaturated carboxylic acid esters. Findeisen in U.S. Pat. No. 4,267,365 discloses a process for the preparation of certain oligomeric acrylic acids wherein the acrylic acid is heated in the presence of 0.001 to 1% by weight of a polymerization inhibitor consisting of molecular oxygen, nitric oxide, a phenol, a quinone, an aromatic amine, a nitro compound or diphenylpicrylhydrazyl to a temperature from about 50.degree. to 200.degree. C. Clonce et al. in U.S. Pat. No. 4,480,116 discloses an improved method for preparing and processing readily polymerizable acrylate monomers by employing phenyl-para-benzoquinone, 2,5-diphenyl-para-benzoquinone, or a mixture thereof. None of these prior art references recognizes the unique mixture of hydroxylamine and phenylenediamine or derivatives thereof having at least one N--H group as desirable for inhibiting acrylonitrile polymerization.